1. Field of the Invention
In one aspect this invention relates to a method of testing ceramic materials. In yet a further aspect, this invention relates to a method to predict and rank the abrasion characteristics of ceramic materials. In still a further aspect, this invention relates to an apparatus for measuring subsurface cracks in opaque ceramic materials. In yet a further aspect this invention relates to the machining characteristics of ceramic materials.
2. Prior Art
Advanced ceramics are useful in a wide variety of wear applications. Some examples are bearings in automotive valve trains and high speed machine tools. The general physical properties of ceramics, such as strength and hardness, are easily measured; however, these properties do not correlate well with the actual bearing performance or machining characteristics of the materials. Lack of a reliable technique for predicting abrasion performance requires each ceramic to be extensively tested under actual operating conditions. This is expensive and time consuming for each material and many inappropriate materials will be tested further increasing costs. Also the time consumed testing unsuitable materials will increase the time until acceptable materials are found for a given application.
A more reliable analysis tool than simply ranking materials by physical properties such as strength is needed for predicting a ceramic material's wear resistance.
It is generally observed that ceramic bearings fail by spalling on the bearing surface. Since spalling is the apparent failure mode, a method for viewing the spalling pattern would provide an improved provide an improved ability to predict and rank the performance of ceramic materials.
The spalling failure mechanism is closely tied to the formation of subsurface lateral cracks in the material, and subsequent propagation of the subsurface lateral crack until a chip falls from the surface. Thus, a technique for investigating the initiation and propagation of lateral cracks in a ceramic will provide an improved means of rating ceramics for wear resistance. The fracture toughness of a ceramic is related to the crack length produced by a defined force causing an indentation.
Because most ceramics suitable for industrial use are opaque and have poor electrical conductivity there was no reliable technique for studying subsurface phenomena since conventional optical and electrical techniques were unavailable.